This invention relates to heat exchangers, and more specifically, to improved side pieces for heat exchangers; as well as methods of making a heat exchanger.
Many heat exchangers in use today, as, for example, vehicular radiators, oil coolers, and charge air coolers, are based on a construction that includes two spaced, generally parallel headers which are interconnected by a plurality of spaced, parallel, flattened tubes. Located between the tubes are thin, serpentine fins. In the usual case, the side most tubes are located just inwardly of side plates on the heat exchanger and serpentine fins are located between those side most tubes and the adjacent side plate.
The side plates are typically, but not always, connected to the headers to provide structural integrity. They also play an important role during the manufacturing process, particularly when the heat exchanger is made of aluminum and components are brazed together or when the heat exchanger is made of other materials and some sort of high temperature process is involved in the assembly process.
More particularly, conventional assembly techniques involve the use of a fixture which holds a sandwiched construction of alternating tubes and serpentine fins. The outside of the sandwich, that is the outer layers which eventually become the sides of the heat exchanger core, is typically provided with side pieces whose ends are typically connected mechanically to the headers. Pressure is applied against the side pieces to assure good contact between the serpentine fins and the tubes during a joining process such as brazing to assure that the fins are solidly bonded to the tubes to maximize heat transfer at their points of contact. If this is not done, air gaps may be located between some of the crests of the fins and the adjacent tube which adversely affect the rate of heat transfer and durability, such as the ability to resist pressure induced fatigue and to withstand elevated pressures.
At the same time, when the heat exchanger is in use, even though the side plates may be of the same material as the tubes, because a heat exchange fluid is not flowing through the side plates but is flowing through the tubes, the tubes will typically be at a higher temperature than the side plates, at least initially during the start up of a heat exchange operation.
This in turn results in high thermal stresses in the tubes and headers. Expansion of the tubes due to relatively high temperatures tends to push the headers apart while the side plates, at a lower temperature, tend to hold them together at the sides of the core. All too frequently, this severe thermal stress in the heat exchanger assembly results in fracture or the formation of leakage openings near the tube to header joints which either requires repair or the replacement of the heat exchanger.
It has been proposed to avoid this problem, after complete assembly of the heat exchanger, by sawing through the side plates at some location intermediate the ends thereof so that thermal expansion of the tubes is accommodated by the side plates, now in multiple sections, which may move relative to one another at the saw cut. However, this solution adds an additional operation to the fabrication process and consequently is economically undesirable.
It is the principal objection of the invention to provide a new and improved heat exchanger, and a method of making the same, that eliminates heat exchanger failure problems due to thermally induced stresses resulting from a difference in thermal expansion between the expansion of the tubes of the heat exchanger and the expansion of the side pieces thereof. It is also an object of the invention to provide a method of making such a heat exchanger.
According to one facet of the invention, there is provided a heat exchanger that includes a pair of spaced, generally parallel headers, a plurality of spaced, generally parallel tubes extending between and in fluid communication with the interior of the headers, a pair of elongated side plates, one at each side of the heat exchanger, and extending between and connected to the headers and spaced from the adjacent tube at the corresponding side of the heat exchanger. Serpentine fins are disposed between adjacent tubes as well as between the side plates and the tubes adjacent thereto. The invention contemplates the improvement wherein each side plate includes at least one opening between its edges which has a periphery with a part of the periphery in close proximity to at least one of the edges along with a score line in each side plate that extends from the part of the periphery to the edge. As a consequence, thermally induced stress will cause the side piece to sever at the location of the opening and the score line and thereafter the stress at tube to header joints or the like is relieved on a permanent basis.
According to one embodiment of the invention, the opening is elongated and is at an acute angle to the direction of elongation of the corresponding side plate.
A preferred embodiment contemplates that the side plate is formed as a channel having a base with at least one leg extending therefrom terminating at the one edge. The opening is formed in the base and in the one leg and the score line is in the one leg.
In a highly preferred embodiment, there is provided a heat exchanger generally as stated previously with the invention contemplating the improvement wherein each side plate is channel shaped having abase and two spaced outstanding legs extending from the base and terminating at opposite edges. First and second, elongated openings are disposed in each of the side plates in side by side relation and have respective center lines that intersect in an acute angle. Each of the openings has a periphery that includes a first part in close proximity to a corresponding one of the edges and a second part in close proximity to the other of the openings. The first and second parts are spaced from one another in the direction of elongation of the side piece and a first line of weakening is located at each of the first parts and extends between a corresponding one of the openings and the adjacent one of the edges. A second line of weakening extends between the openings at the second part.
As a consequence of this construction, the side plates may sever at the lines of weakening to relieve stresses as before. Moreover, the formation of the side plate as a channel and the effective staggering of the first and second lines of weakening does not materially reduce the bending strength or resistance to bending of the side plate so that a high degree of structural integrity is maintained prior to assembly of the side plates to the core. This accomplishes two purposes: It allows handling of the side pieces prior to assembly to the core without requiring great care on the part of the persons that must handle the side pieces; and it distributes the expansion stress that otherwise would be focused at the location of severance.
In a preferred embodiment, the acute angle is on the order of 90xc2x0.
Preferably, the first and second lines of weakening are defined by V-shaped notches in the legs and in the base of the channel respectively.
In a highly preferred embodiment, the lines of weakening have a length of about 4.6 mm or less.
The invention also contemplates the provision of a method of making an aluminum heat exchanger which includes the steps of: (a) assembling the components of a heat exchanger core in a fixture to have spaced headers, spaced tubes extending between the headers, side plates extending between the headers at the sides of the core and serpentine fins located between adjacent tubes as well as between the side plates and the adjacent tube at each side of the core; (b) mechanically fixing each end of the side plate to the adjacent header; (c) prior to steps (a) and (b), weakening the side plate at a point intermediate its ends so as to reduce its ability to withstand tension while not materially affecting its ability to withstand bending; and (d) subjecting the assembly resulting from step (b) to brazing temperatures to (i) braze the components together and (ii) allow the severance of each side piece at said point as a result of thermally induced stress. Whether services will in fact occur generally depends on the rate at which the brazed components cool following brazing.
Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.